This invention relates to a battery vent cap adapted to vent gases generated during cell operation and prevent the escape of electrolyte foam. More particularly, this invention relates to a vent cap comprising strategically located reservoir of an antifoam agent for dissipating electrolyte-laden foam attempting to escape from the battery through the cap, while readily venting foam-free gases from the cell.
The elements of a zinc-nickel oxide secondary electrochemical cell and aqueous alkaline electrolyte are typically arranged within a container that is sealed except for a vent provided to release gases formed during cell operations, particularly charging. The vent is preferably through a removable cap mounted in the container top, which also provides access to the cell for adding water to replenish the electrolyte. Normally, the gases bubble up through the electrolyte and are vented to the ambient atmosphere. However, wetting agents included in certain cell elements may leach into the alkaline electrolyte. These agents alter the surface activity of the electrolyte so that the bubbles do not break up, but instead form a tenacious foam. This electrolyte foam seeps through the vent and is lost out the cell. The escaped foam corrodes electrical connections to the container or creates voltage shunts that reduce the cell output.
One method for preventing foaming is to add a silicone-base antifoam agent to the electrolyte. The antifoam agent dissipates the foam without significant interference with cell operations. However, the agent loses its effectiveness after a short time, apparently because the alkaline electrolyte decomposes the silicone-base compound. Thus, periodic additions are required to prevent foam from escaping out the cell. Because of the very short time that the agent remains effective, typically only several minutes, these additions have not been practical for a cell in normal field use.
Therefore, it is an object of this invention to provide a device that is adapted to be mounted in a container for an electrochemical cell for venting gases therefrom while preventing the escape of electrolyte carried along with the gases as a foam. The device is adapted to contain a reservoir of antifoam agent that is strategically located to dissipate the foam before it escapes the cell and to thereby free the gases for venting.
It is a further object of this invention to provide a vent device for an electrochemical cell having an aqueous alkaline electrolyte, which device comprises a reservoir that is adapted to contain a silicone-base antifoam agent away from contact with the electrolyte and to intermittently dispense minute amounts of the agent into the foam to at least partially dissipate it. The device thus provides foam-free venting of gases from the cell over an extended time suitable for normal field use.